Read sfh chi output files

Hess planes for number of model stars; number of observed stars,
and chi-per-pixel are returned. This is done in the hess.py module.

Also:

- Refactor pix list reshaping into reshape_pixarray
- Read chi files with read_chi
- Refactor extent into compute_cmd_extent
Fixes for Hess plots of synth cmds
Create ChiTriptykPlot for vis solution in Hess

This plot has three panels showing the output from the sfh program:
the observed Hess diagram, the modelled Hess diagram, and a chi
Hess diagram.